Flame ionization detectors are known in the art and are frequently employed to further analyze a sample which is an effluent of a gas chromatograph. A review of the operating characteristics of flame ionization detectors by Blades is found at J. Chromatographic Science, 11: 251-255 (May 1973).
Flame ionization detectors presently used for the detection of traces of hydrocarbons (i.e. organic compounds containing CH groups) in gaseous samples are based on chemi-ionization reactions which occur when a hydrocarbon is introduced within a hydrogen/oxygen or hydrogen/air flame [McWilliam et al, Gas Chromatography, 1958, (Desty et al., ed.) Academic Press, N.Y., 1958, pp. 142-145; Perry, Introduction to Analytical Gas Chromatography, Marcel Dekker, Inc., N.Y., 1981, pp. 156-177; Littlewood, Gas Chromatography, Principles, Techniques and Applications, Academic Press, N.Y., 1970, pp. 301-307; Bruderreck et al., J. Chromatog., 1964, 36: 461-473]. The primary reaction thought to be responsible for this chemi-ionization is that between the CH radical and 0 atoms [Calcote, Combust. Flame, 1957, 1: 385, i.e., EQU CH(X.sup.2 .pi.)+0.fwdarw.HCO.sup.+ +e.sup.-. (1)
Present hydrocarbon detection methods are based upon measurement of the ionization current drawn from the flame by electrodes polarized by external circuitry. Ionization detection efficiencies approaching n.sub.c .times.10.sup.-4 electrons detected per hydrocarbon molecule, where n is the number of carbon atoms in the molecule, are possible with currently available flame ionization detectors.
In my laboratory it was recently found that the chemi-ionization reactions EQU CH(A.sup.2 .DELTA.,B.sup.2 .SIGMA..sup.-)+0.fwdarw.HCO.sup.+ +e.sup.-( 2)
have rate constants, under flame conditions, which are approximately 2000 times faster than that previously determined [Peeters et al., 15th Symposium (Internat.) on Combustion, the Combustion Inst., 1975, pp. 696.] for the chemi-ionization reaction (1). Laser absorption methods can be used to convert ground electronic state CH(X.sup.2 .pi.) radicals to electronically excited CH(A.sup.2 .DELTA., B.sup.2 .SIGMA..sup.-) radicals so that chemi-ionization can proceed at a vastly accelerated rate via reaction (2) compared with the chemi-ionization rate associated with reaction (1). This laser-induced enhancement of chemi-ionization provides the basis for a new type of flame ionization detector described herein.